The presently disclosed embodiments are directed to an imaging member used in electrostatography. More particularly, the disclosure embodiments pertain to the preparation of an improved electrophotographic imaging member having a protective overcoat layer comprising a low surface energy polymeric material to enhance the imaging member physical/mechanical function as well as render its service life extension and a process for making and using the member.
In electrostatographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Electrostatographic imaging members are well known in the art. Typical electrostatographic imaging members include, for example: (1) electrophotographic imaging member (photoreceptors) commonly utilized in electrophotographic (xerographic) processing systems; (2) electroreceptors such as ionographic imaging member belts for electrographic imaging systems; and (3) intermediate toner image transfer members such as an intermediate toner image transferring member which is used to remove the toner images from a photoreceptor surface and then transfer the very images onto a receiving paper.
Although the scope of the present invention covers the preparation of all types of flexible electrostatographic imaging members in either a rigid drum design or a flexible belt configuration, however for reason of simplicity, the embodiments and discussion thus followed hereinafter will be focused solely on and represented by electrophotographic imaging members in the flexible belt configuration. Electrophotographic flexible belt imaging members may include a photoconductive layer including a single layer or composite layers. The flexible belt electrophotographic imaging members may be seamless or seamed belts; and seamed belts are usually formed by cutting a rectangular sheet from a web, overlapping opposite ends, and welding the overlapped ends together to form a welded seam. Typical electrophotographic imaging member belts include a charge transport layer (CTL) and a charge generating layer on one side of a supporting substrate layer and an anticurl back coating coated onto the opposite side of the substrate layer. By comparison, a typical electrographic imaging member belt does, however, have a more simple material structure; it includes a dielectric imaging layer on one side of a supporting substrate and an anti-curl back coating on the opposite side of the substrate to render flatness. Since typical negatively-charged flexible electrophotographic imaging members exhibit undesirable upward imaging member curling after completion of coating the top outermost charge transport layer, an anticurl back coating, applied to the backside, is required to balance the curl. Thus, the application of anticurl back coating is necessary to provide the appropriate imaging member with desirable flatness.
One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a negatively-charged photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer (CTL). Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous CTL and the supporting conductive layer. Alternatively, the CTL of a positively-charged imaging member is sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
In the case where the charge generating layer (CGL) is sandwiched between the outermost exposed CTL and the electrically conducting layer, the outer surface of the CTL is charged negatively and the conductive layer is charged positively. The CGL then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the CTL. In the alternate case when the CTL is sandwiched between the CGL and the conductive layer, the outer surface of Gen layer is charged positively while conductive layer is charged negatively and the holes are injected through from the CGL to the CTL. The CTL should be able to transport the holes with as little trapping of charge as possible. In a typical flexible imaging member web like photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers used in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, an optional blocking layer, an optional adhesive layer, a charge generating layer, a CTL and a conductive ground strip layer adjacent to one edge of the imaging layers, and an optional overcoat layer adjacent to another edge of the imaging layers. Such a photoreceptor usually further comprises an anticurl back coating layer on the side of the substrate opposite the side carrying the conductive layer, support layer, blocking layer, adhesive layer, charge generating layer, CTL and other layers.
Typical negatively-charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer (CGL), and a charge transport layer (CTL). The CTL is usually the last layer to be coated to become the outermost exposed layer and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 115° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered photoreceptor material is obtained after finishing the CTL coating through drying/cooling process, upward curling of the multilayered photoreceptor is observed.
This upward curling is a consequence of thermal contraction mismatch between the CTL and the substrate support. Since the CTL in a typical photoreceptor device has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the CTL exhibits a larger dimensional shrinkage than that of the substrate support as the imaging member web stock (after through elevated temperature heating/drying process) as it cools down to ambient room temperature. The exhibition of upward imaging member curling after completion of CTL coating is due to the consequence of the heating/cooling processing, according to the mechanism: (1) as the web stock carrying the wet applied CTL is dried at elevated temperature, dimensional contraction does occur when the wet CTL coating is losing its solvent during 115° C. elevated temperature drying, because the CTL at 115° C. still remains as a viscous liquid after losing its solvent. Since its glass transition temperature (Tg) is about 85° C., the CTL will flow to automatically re-adjust itself to compensate the losing of solvent and maintain its dimension; (2) as the CTL in a viscous liquid state is cooling down further and reaching its Tg at 85° C., the CTL instantaneously solidifies and adheres to the CGL because it has transformed itself from being a viscous liquid into a solid layer at its Tg; and (3) cooling down the solidified CTL of the imaging member web from 85° C. down to 25° C. room ambient will then cause the CTL to contract more than the substrate support since it has an approximately 3.7 times greater thermal coefficient of dimensional contraction than that of the substrate support. This dimensional contraction mis-match results in tension strain built-up in the CTL, at this instant, is pulling the imaging member upward to exhibit curling. If unrestrained at this point, the imaging member web stock will spontaneously curl upwardly into a 1.5-inch tube. To offset the curling, an anticurl back coating is applied to the backside of the flexible substrate support, opposite to the side having the charge transport layer, and render the imaging member web stock with desired flatness.
Curling of a photoreceptor web is undesirable because it hinders fabrication of the web into cut sheets and subsequent welding into a belt. An anticurl back coating having a counter curling effect equal to and in the opposite direction to the applied layers is applied to the reverse side of the active imaging member to eliminate the overall curl of the coated device by offsetting the curl effect which is arisen from the mismatch of the thermal contraction coefficient between the substrate and the CTL, resulting in greater CTL dimensional shrinkage than that of the substrate. Although the anticurl back coating counters and balances the curl so as to promote the imaging member web to lay flat, nonetheless, common anticurl back coating formulations are not always providing satisfying dynamic imaging member belt performance result under a normal machine functioning condition; for example, exhibition of anticurl back coating wear and its propensity to cause electrostatic charging-up are the frequently seen problems to prematurely cut short the service life of a belt and requires its frequent costly replacement in the field.
Other layers of the imaging member, say for example the top outermost exposed CTL in particular of a negatively charge imaging member, do also subjected to and suffer from the machine operational conditions, such as exposure to high surface friction and extensive cycling. Such harsh conditions lead to wearing away and susceptibility of surface scratching of the CTL which otherwise adversely affect machine performance. Another imaging member functional problem associated with the CTL is its propensity to give rise to early development of surface filming due its high surface energy; CTL surface filming is undesirable because it does pre-maturely cause degradation of copy printout quality. Moreover, the outermost exposed CTL is also been found to exhibit early onset of surface cracking, as consequence of repetition of bending stress belt cyclic fatiguing, airborne chemical species exposure, and direct solvent contact, under a normal machine belt functioning condition. CTL cracking is a serious mechanical failure since the cracks do manifest themselves into defects in print-out copies. All these imaging member layers failures are major issues remained to be resolved, because they pre-maturely cut short the functional life of an imaging member and prevent it from reaching the belt life target; early imaging member functional failure does thereby require its frequent costly replacement in the field.
In U.S. Pat. No. 5,069,993, an exposed layer in an electrophotographic imaging member is provided with increase resistance to stress cracking and reduced coefficient of surface friction, without adverse effects on optical clarity and electrical performance. The layer contains a polymethylsiloxane copolymer and an inactive film forming resin binder. Various specific film forming resins for the anti-curl layer and adhesion promoters are disclosed.
U.S. Pat. No. 5,021,309 shows an electrophotographic imaging device, with material for an exposed anti-curl layer has organic fillers dispersed therein. The fillers provide coefficient of surface contact friction reduction, increased wear resistance, and improved adhesion of the anti-curl layer, without adversely affecting the optical and mechanical properties of the imaging member.
U.S. Pat. No. 5,919,590 shows An electrostatographic imaging member comprising a supporting substrate having an electrically conductive layer, at least one imaging layer, an anti-curl layer, an optional ground strip layer and an optional overcoat layer, the anti-curl layer including a film forming polycarbonate binder, an optional adhesion promoter, and optional dispersed particles selected from the group consisting of inorganic particles, organic particles, and mixtures thereof.
In U.S. Pat. No. 4,654,284 an electrophotographic imaging member is disclosed comprising a flexible support substrate layer having an anti-curl layer, the anti-curl layer comprising a film forming binder, crystalline particles dispersed in the film forming binder and a reaction product of a bifunctional chemical coupling agent with both the binder and the crystalline particles. The use of VITEL PE 100 in the anti-curl layer is described.
In U.S. Pat. No. 6,528,226 a process for preparing an imaging member is disclosed that includes applying an organic layer to an imaging member substrate, treating the organic layer and/or a backside of the substrate with a corona discharge effluent, and applying an overcoat layer to the organic layer and/or an anticurl back coating to the backside of the substrate.
The above prior art disclosures show that, while attempts to resolve CTL and anticurl back coating failures described above have been successful with providing a solution, often times the success is negated due to the creation of another set of problems. Therefore, there is an urgent need to provide improved imaging members that have mechanically robust outer layers to effect service life extension but without causing the introduction of other undesirable problems.
A number of current flexible electrophotographic imaging member belts are multilayered photoreceptor belts that, in a negative charging system, comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer (CGL), a charge transport layer (CTL), and an optional anticurl back coating at the opposite side of the substrate support to render flatness. In such an imaging member belt design, the CTL is therefore the top outermost exposed layer. In a typical machine design, a flexible imaging member belt is mounted over and around a belt support module comprising numbers of belt support rollers, such that the top outermost CTL is exposed to all electrophotographic imaging subsystems interactions and charging devices chemical emission attack. Under normal machine electrophotographic imaging and cleaning operating conditions, the top exposed CTL surface of the flexible imaging member belt is constantly subjected to physical/mechanical/electrical/chemical species interactions, such as for example, the mechanical sliding actions of cleaning blade and cleaning brush, electrical charging devices corona effluents exposure, developer components, image formation toner particles, hard carrier particles, debris and loose CaCO3 particles from receiving paper, and the like during dynamic belt cyclic motion. These interactions against the surface of the CTL have been found to cause surface scratching, abrasion, and rapid CTL surface wear; in some instances, the CTL wears away by as much as 10 micrometers after approximately 20,000 dynamic belt imaging cycles. Excessive CTL wear is a serious problem because it causes significant change in the charged field potential and adversely impacts copy printout quality. Another consequence of CTL wear is the decrease of CTL thickness alters the equilibrium of the balancing forces between the CTL and the anti-curl back coating and impacts imaging member belt flatness. The reduction of the CTL by wear causes the imaging member belt to curl downward at both edges. Edge curling in the belt is an important issue because it changes the distance between the belt surface and the charging device(s), causing non-uniform surface charging density which manifests itself as a “smile” print defect on paper copies. Such a print defect is characterized by lower intensity of print-images at the locations over both belt edges. The susceptibility of the CTL surface to scratches (caused by interaction against developer carrier beads and the hard CaCO3 particles and debris from paper) has also been identified as a major imaging member belt functional failure since these scratches do manifest themselves as print defects in paper copies.
Moreover, since the current CTLs have a high surface energy of about 39 dynes/cm. The surface of the CTL is therefore prone to collect toner residues, dirt/debris particles, and additives from receiving papers. The eventual fusion of these collected species causes the formation of comets and filming over the outer surface of the CTL, further degrading the image quality of printouts. Another problem associated with high surface energy is that it also impedes the cleaning blade and cleaning brush function.
In a rigid electrophotographic imaging member drum utilizing a contact AC Bias Charging Roller (BCR), it has been found that ozone species attack on the CTL polymer binder is more pronounced because of the close vicinity of the BCR to the CTL of the imaging member drum.
The early exhibition of CTL failure (occurred in the imaging member of either in a flexible belt configuration or as a rigid drum design) does significantly cut short the intended service life of the imaging member and thereby requires frequent costly imaging member replacement in the field.
Thus, electrophotographic imaging members comprising a supporting substrate, having a conductive surface on one side, coated over with at least one photoconductive layer and coated on the other side of the supporting substrate with a conventional prior art anticurl back coating that does exhibit deficiencies which are undesirable in advanced automatic, cyclic electrophotographic imaging copiers, duplicators, and printers. While the above mentioned electrophotographic imaging members may be suitable or limited for their intended purposes, further improvement on these imaging members are desirable and urgently needed. For example, there continues to be the need for improvements in such systems, particularly for an imaging member belt that includes a mechanical robust, filming-free, and scratch resistant surface which sufficiently counters curling to render flatness, reduces friction, has superb wear resistance, provides lubricity to ease belt drive, nil or no wear debris, and eliminates electrostatic charge build-up problem, even in larger printing apparatuses.
According to aspects illustrated herein, there is provided an overcoating for an overcoat layer that addresses the shortcomings of traditional imaging layers discussed above. The present application is related to commonly assigned U.S. patent application Ser. No. 11/199,842, filed Aug. 9, 2005, entitled “Anti-curl Backing Layer for Electrostatographic Imaging Members,” commonly assigned U.S. patent application Ser. No. 11/220,777, filed Sep. 7, 2005, entitled “Flexible Imaging Member with Improved Anticurl Back Coating,” commonly assigned U.S. patent application Ser. No. 11/227,639, filed Sep. 15, 2005, entitled “Anticurl Backing Layer for Electrophotographic Imaging Members,” and commonly assigned U.S. patent application Ser. No. 11/315,800, filed Dec. 22, 2005, entitled “Imaging Member,” and commonly assigned U.S. Patent Application entitled “IMAGING MEMBER HAVING ADJUSTABLE FRICTION ANTICURL BACK COATING,” to Yu et al. Ser. No. 11/471,471, filed on Jun. 20, 2006, which are all herein incorporated by reference. While the above applications provide anticurl back coatings that address the shortcomings of traditional anticurl back coatings, there is still a further need for improvements in the mechanical robustness of other imaging member layers.